1. Field of the Invention
The present invention relates in general to a by-pass solenoid for a variable bleed flow system and method, and more particularly to a by-pass solenoid with an integral check valve for a hydraulic system and method for by-passing or bleeding excess flow of hydraulic fluid while maintaining a constant pressure.
2. Description of the Related Art
Solenoid actuated hydraulic valves are used in many types of machines and applications to control the flow of fluid (hydraulic or pneumatic) to and from fluid pressure devices, for example, hydraulic cylinders, hydraulic motors, pumps, etc. The valves are controlled by circuits which selectively energize the various solenoids in accordance with an intended operating sequence. Solenoid valves may be of the ON/OFF variety operating as either normally open or normally closed, or the solenoid valve may be a variable (proportional) force solenoid valve. A variable force solenoid valve is controlled by an electrical signal providing variable voltage or controlled current to provide an increasing or decreasing magnetic force on an armature for effecting movement of a spool valve. Movement of the spool valve results in an increase or decrease in fluid flow.
Solenoid valves have found recent application in automatic transmissions and power steering systems on some vehicles. The fluid pressure and/or flow is controlled to provide smooth gear shifting in the transmission, or to provide good steering wheel "feel" in power assist steering.
In electronic control applications for automatic transmissions, variable force solenoid valve assemblies are used to provide control over fluid pressures which are provided in response to electrical input signals supplied to the variable force solenoid assemblies by an engine control unit (ECU). The variable force solenoid valve assemblies are employed to regulate fluid control devices which cause the "shift-feel" of an automatic transmission.
Once such type of variable force solenoid valve is disclosed in U.S. Pat. No. 5,853,028 assigned to the Assignee of the present invention and incorporated herein by reference. U.S. Pat. No. 5,853,028 describes an improved variable force solenoid valve with a spool valve that experiences a minimum of oscillations and instability.
In an electronically controlled automatic transmission or a continuously variable transmission (CVT), it has become desirable to move the pump out from inside the transmission to a location where it may be operated directly from the engine of the vehicle. Since the engine speed will directly effect pump flow, a by-pass system is required to divert excess flow during high engine speed while maintaining a desired pressure. While a continuously variable pump may be employed for this type of application, a continuously variable pump is more expensive than a fixed displacement pump. So, it would be preferred to use the fixed displacement pump for cost reasons.
Consequently, there is a need for a simple, reliable hydraulic system and method to by-pass excess hydraulic fluid flow in a hydraulic circuit which can maintain a fairly constant fluid flow pressure. There exists a need for a fluid flow control device employed in the hydraulic system which allows return of excess fluid flow to the system reservoir, or allows re-circulation of excess fluid back to the pump inlet. The fluid flow control device would ideally offer a smaller size advantage along with being an integral flow control device.